Locking device for locking a rod-shaped element in a receiving part of a bone anchor and bone anchor with such a locking device

ABSTRACT

A locking device includes a first locking member including a first end, a second end, a central axis, an outer surface with an external thread, a coaxial bore, and an internal thread provided along at least a portion of the bore, and a second locking member including a first end, a second end, and an outer surface with an external thread. When the second locking member is positioned in the first locking member, the second locking member is movable along the central axis by rotating the second locking member. Movement of the second locking member is limited towards the first end of the first locking member by a first stop and towards the second end of the first locking member by a second stop, where at least one of the first stop or second stop is arranged in the bore of the first locking member.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No. 13/564,261, filed Aug. 1, 2012, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/515,635, filed Aug. 5, 2011, the contents of which are hereby incorporated by reference in their entirety, and which claims priority to European Patent Application EP 11 176 723.2, filed Aug. 5, 2011 the contents of which are hereby incorporated by reference in their entirety.

BACKGROUND

1. Field of the Invention

The invention relates to a locking device for securing a rod-shaped element in a receiving part of a bone anchor and to a bone anchor with such a locking device. The locking device includes a first locking element and a second locking element that is positionable in the first locking element and that is protected against backing out or unintended removal from the first locking element. The invention is particularly applicable to a polyaxial bone screw, where a head of a screw element and the rod-shaped element can be fixed independently from one another.

2. Description of Related Art

A polyaxial bone screw wherein the head of the screw element and the rod can be fixed independently is known, for example, from U.S. Pat. No. 7,223,268 B2. The locking device described therein has a first locking element cooperating with a thread provided at the inner wall of the receiving part and a second locking element in the form of a set screw that is contained in the first locking element.

A nested fastener and set screw combination for securing a spinal fixation rod to a bone screw, is described in U.S. Pat. No. 7,204,838 B2. The fastener base has a central threaded bore to receive a threaded set screw. The fastener base is provided with a radially inwardly extending abutment shoulder to engage and abut the set screw, prohibiting advancement of the set screw out of a top of the fastener and allowing for a simultaneous removal of the set screw and the fastener.

SUMMARY

It is an object of the invention to provide an improved locking device, and a bone anchor with such a locking device, that exhibits enhanced safety during manipulation in the course of a surgery.

In the locking device, a second locking element is contained or held in a first locking element in such a way that the second locking element is not separable from the first locking element during use. Therefore, if an operating error with regard to an instrument for inserting the locking device or other occurrence happens, or when the second locking element is loosened again during further adjustments of a position of a rod, the second locking element will not escape or fall out of the first locking element and therefore cannot get lost.

In some cases, the locking device according to embodiments of the invention can be delivered in a pre-assembled manner. Because the second locking element is contained in the first locking element in a manner protected against backing out or loss, handling during surgery is more effectively facilitated and security is enhanced.

Although the locking device according to embodiments of the invention is particularly applicable to a polyaxial bone anchor that is configured to allow independent head and rod fixation, it can also be used with monoaxial bone anchors, with bone plates having bone screws with variable angle placement, or with other devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent from the description of embodiments by means of the accompanying drawings. In the drawings:

FIG. 1 shows a perspective exploded view of an embodiment of a polyaxial bone screw with a first embodiment of a locking device;

FIG. 2 shows a perspective view of the polyaxial bone screw with the locking device of FIG. 1 in an assembled state;

FIG. 3 shows a cross-sectional view of a first locking element of the locking device according to the first embodiment, the section being taken along a plane containing a central axis;

FIG. 4 shows a perspective view from a top of the first locking element of FIG. 3;

FIG. 5 shows a perspective view from a bottom of the first locking element of FIG. 3;

FIG. 6 shows a cross-sectional view of a second locking element of the locking device according to the first embodiment, the section taken along a plane containing a central axis;

FIG. 7 shows a perspective view from a top of the second locking element of FIG. 6;

FIG. 8 shows a perspective view from a bottom of the second locking element of FIG. 6;

FIGS. 9 to 11 show steps of assembling the locking device according to the first embodiment, in cross-sectional views, the sections being taken in a plane containing a central axis of the locking device;

FIG. 12 a shows a cross-sectional view of the locking device of the first embodiment with a portion of a tool in a first position, the section being taken in a plane containing the central axis of the locking device;

FIG. 12 b shows an enlarged portion of FIG. 12 a;

FIG. 13 a shows a cross-sectional view of the first embodiment of the locking device with a portion of a tool in a second position, the section being taken in a plane containing the central axis of the locking device;

FIG. 13 b shows an enlarged portion of FIG. 13 a;

FIG. 14 shows a cross-sectional view of the polyaxial bone screw of FIG. 1 with the locking device according to the first embodiment and a portion of the tool in an assembled state, the section being taken in a plane containing the central axis of the locking device and an axis of a screw element;

FIG. 15 shows a cross-sectional view of the polyaxial bone screw with the locking device according to the first embodiment without the tool;

FIG. 16 shows a perspective exploded view of a locking device according to a second embodiment;

FIGS. 17 to 19 show steps of assembling the locking device according to the second embodiment in a perspective view;

FIG. 20 shows a perspective view from a top of a first locking element of the locking device according to the second embodiment;

FIG. 21 shows a cross-sectional view of the first locking element of FIG. 20, the section taken in a plane containing a central axis;

FIG. 22 shows a cross-sectional view along line A-A of the first locking element shown in FIG. 21;

FIG. 23 shows a cross-sectional view of a second locking element of the locking device according to the second embodiment, the section being taken in a plane containing a central axis;

FIG. 24 shows a top view of the second locking element shown in FIG. 23;

FIG. 25 shows a perspective view of the second locking element of FIG. 23;

FIG. 26 shows a cross-sectional view of the locking device according to the second embodiment with the second locking element in a first position, the section being taken in a plane containing the central axis of the locking device;

FIG. 27 shows a cross-sectional view of the second locking device shown in FIG. 26 along line B-B of FIG. 26;

FIG. 28 shows a cross-sectional view of the locking device according to the second embodiment with the second locking element in a second position, the section being taken in a plane containing the central axis of the locking device;

FIG. 29 shows a cross-sectional view of the locking device of FIG. 28 along line D-D of FIG. 28;

FIG. 30 shows a cross-sectional view of the second locking element according to a further modified embodiment;

FIG. 31 shows a perspective view from a top of the second locking element of FIG. 30;

FIG. 32 shows a top view of the second locking element of FIG. 30;

FIG. 33 shows a perspective view from the bottom of the second locking element of FIG. 30;

FIG. 34 a shows a cross-sectional view of a locking device according to the further modified embodiment;

FIG. 34 b shows an enlarged portion of FIG. 34 a;

FIG. 35 a shows another cross-sectional view of the locking device according to the further modified embodiment; and

FIG. 35 b shows an enlarged portion of FIG. 35 a.

DETAILED DESCRIPTION

An embodiment of a polyaxial bone screw or bone anchor is shown in FIGS. 1 and 2. The polyaxial bone screw includes a screw element 1 with a threaded shank 2 and a head 3. The head 3 has the shape of a segment of a sphere and has at its free end a recess 3 a for engagement with a screwdriver. The screw element 1 is pivotably held in a receiving part 4. The receiving part 4 is substantially cylindrical with a first end 41 and a second end 42, and a seat for the head 3 near the second end 42, which will be described later. A coaxial bore 43 extends from the first end 41 in the direction of the second end 42. Furthermore, a U-shaped recess 44 is provided at the first end 41, by means of which two free legs 45, 46 are formed. An internal thread 47 is provided at an inner wall of said legs 45, 46. The U-shaped recess 44 and the legs 45, 46 form a channel for receiving a rod 100. A pressure element 5 is provided that is configured to exert pressure onto the head 3 for locking the head 3 in a particular angular orientation of the screw element 1 relative to the receiving part 4. The pressure element 5 is substantially cylindrical with a first end 51 and an opposite second end 52, a coaxial bore 53, and a U-shaped recess 54 extending from the first end 51 into the direction of the second end 52. By means of the U-shaped recess 54, two free legs 55, 56 are formed that also form a channel for receiving the rod 100. A height of the legs 55, 56 in the embodiment shown is greater than a diameter of the rod 100, so that the legs 55, 56 extend above the rod 100 when the rod 100 is inserted.

The bone anchor further includes a locking device, which includes a first locking element 6 and a second locking element 7. In an assembled state, the second locking element 7 is nested within the first locking element 6, and is movable between a first position and a second position in an axial direction along a central axis C.

The locking device according to the first embodiment will now be described in more detail with reference to FIGS. 3 to 11. FIGS. 3 to 5 show the first locking element 6 before assembly with the second locking element 7. The first locking element 6 is substantially cylindrical and has a first end 61 and a second end 62. At least a portion of an outer surface of the first locking element 6 includes an external thread 63 that is configured to cooperate with the internal thread 47 provided on the legs 45, 46 of the receiving part 4. The first locking element 6 is provided with a coaxial bore 64 with a bore axis C, and a portion 65 including an internal thread. The portion 65 is adjacent or near the second end 62 and extends up to a distance from the first end 61. A thread form of the external thread 63 is preferably a thread form that prevents splaying of the legs 45, 46 of the receiving part 4, such as a flat thread (as shown) or a negative angle thread with a negative angle of a load-bearing surface. The internal thread can have any thread form, such as a metric thread as shown for example in FIGS. 9 to 11, and can alternatively have a flat thread.

The first locking element 6 further includes at the second end 62 a threadless concavely rounded hollow cylindrical portion 62 a that can be, after insertion of the second locking element 7, bent inwards as described below. Adjacent the first end 61, the coaxial bore has an inner diameter d₁ that is the same or smaller than an inner diameter d₂ measured between the crests of the thread of the internally threaded portion 65, and smaller than a diameter d₃ between the thread roots of the internally threaded portion 65. By means of this, a stop in the form of an abutment 66 is provided for stopping advancement of the second locking element 7 in the direction of the first end 61 of the first locking element 6. Adjacent the first end 61, an engagement structure 67 for engagement with a tool is also provided. The engagement structure 67 can be a plurality of semi-spherical grooves arranged around the axis C.

The second locking element 7 is also substantially cylindrical, and has a first end 71 and an opposite second end 72. Adjacent the first end 71, a portion 73 with an external thread is provided that is configured to cooperate with the internally threaded portion 65 of the first locking element 6. Adjacent the second end 72, a threadless portion 74 is provided. An outer surface of the portion 74 is substantially smooth. An outer diameter of the threadless portion 74 is smaller than an outer diameter of the threaded portion 73. The second locking element 7 further includes a coaxial bore or recess 75, and a coaxial recess 76 adjacent the first end 71 that has an inner diameter that is greater than that of the recess 75. Furthermore, adjacent the first end 71, an engagement structure in the form of a plurality of longitudinal grooves 77 is provided around a central axis C that serve for engagement with an insertion and/or driving tool. The longitudinal grooves 77 may open into a face of the second end 72, forming circular holes 78 therein. The holes 78 can serve for engagement with a tool to mount the second locking element 7 into the first locking element 6 from the second end 62.

The steps of assembling the first and the second locking elements 6 and 7 will be explained, referring to FIGS. 9 to 11. In a first step, the second locking element 7 is introduced into the first locking element 6 from the second end 62 of the first locking element 6. Then, as shown in FIG. 10, the second locking element 7 is advanced by screwing it in the direction to the first end 61 until the first end 71 of the second locking element 7 abuts against the abutment 66 provided at the coaxial bore of the first locking element 6. A length of the second locking element 7 in an axial direction is such that when the second locking element 7 assumes a first position, wherein its first end 71 abuts against the abutment 66, the second end 72 of the second locking element 7 is located within the concavely rounded hollow cylindrical portion 62 a. Thereafter, as shown in FIG. 11, the concavely rounded hollow cylindrical portion 62 a is bent inwards, for example, until it nearly touches the threadless portion 74 of the second locking element 7. By this operation, a second stop in the form of an annular abutment 62 b is formed. The inner diameter d₄ of the annular abutment 62 b is smaller than the inner diameter d₃ between the roots of the threaded portion 65 of the first locking element 6, but greater than an outer diameter of the threadless portion 74 of the second locking element 7.

All or parts of the polyaxial bone screw, including the locking device, may be made from a biocompatible material. Such biocompatible materials may include, for example, titanium, stainless steel, biocompatible alloys such as titanium nickel alloys, for example Nitinol, or biocompatible plastic materials, such as PEEK (polyetheretherketone). The components can all be made of the same material or of different materials.

The function or operation of the locking device is shown in FIGS. 12 a-13 b. In FIG. 12 a, a tool 101 is shown that includes an inner drive portion 102 and an outer drive portion 103. The inner drive portion 102 engages the recess 76 and the engagement structure 77 of the second locking element 7. The outer drive portion 103 engages the engagement structure 67 in the first locking element 6. The inner drive portion 102 and the outer drive portion 103 can be rotated independently of one another. By means of the inner drive portion 102, the second locking element 7 can be rotated so that it is advanced towards the first end 61 of the first locking device 6 until, as shown in FIGS. 12 a and 12 b, a first thread turn 73 a adjacent the first end 71 of the second locking element abuts against the abutment 66 formed by the inner wall of the coaxial bore 64. Because the inner diameter d₁ of the inner wall of the coaxial bore 64 is smaller than the inner diameter d₃ at the root of the internal thread 63 of the first locking element 6, the second locking element 7 cannot advance further towards the first end 61, and is at a first end position at this point. When the second locking element 7 is in the first end position, a gap 104 may be present between the first end 61 of the first locking element 6 and the outer drive portion 103.

By rotating the first drive portion 102 and by moving down the second drive portion 103 so that it abuts against the first end 61 of the first locking element 6, the second locking element 7 advances in the direction of the second end 62 of the first locking element 6 (i.e., away from the first end 61). The advancement is stopped, when a last thread turn 73 b that is on a side of the second locking element 7 opposite the first thread turn 73 a abuts against the second stop formed by the annular abutment 62 b. Because the inner diameter d₄ of the annular abutment 62 b is smaller than the inner diameter d₃ between the roots of the threaded portion 63 of the first locking element 6, the second locking element 7 can not be screwed further towards the second end 62 or away from the first end 61 and, therefore, can not escape. When the second locking element 7 is in this second end position, the threadless portion 74 projects out of the second end 62 of the first locking element 6.

The use of the locking device according to the first embodiment is shown in FIGS. 14 and 15. FIG. 14 shows the polyaxial bone screw according to FIGS. 1 and 2 in an assembled state. To allow pivoting before fixation, the receiving part 4 has a seat portion 48, wherein the screw head 3 is held. The seat portion can be spherically-shaped. The screw head 3 and the threaded shank 2 extend through an opening 49 provided at the second end 42 of the receiving part 4. The pressure element 5 is arranged on top of the head 3 and may partially encompass the head 3 with a spherically-shaped recess 57. The pressure element 5 and the receiving part 4 are aligned such that the U-shaped recesses 44, 54 are in alignment, to allow insertion of the rod 100. When the rod 100 rests in the U-shaped recess 54 of the receiving part 5, the legs 55, 56 extend above the rod 100.

The locking device may be inserted into the receiving part 4 in a pre-assembled manner, in which the second locking element 7 is contained in the first locking element 6. With the outer drive portion 103, the first locking element 6 is screwed into the internal thread 47 of the receiving part 4 until its second end 62 abuts against upper free surfaces of the legs 55, 56 of the pressure element 5. By further advancement of the first locking element 6, the pressure element 5 exerts pressure onto the head 3 until the head 3 is finally locked in a desired angular position of the threaded shank 2 relative to the central axis C.

The second locking device 7 is advanced by rotating it with the inner drive portion 102 until the threadless portion 74 extends out of the first locking element 6. Thereby, the second end 72 of the second locking element engages the surface of the rod 100 and presses the rod 100 into the U-shaped recess 54 of the pressure element 5 and/or the U-shaped recess 44 of the receiving part 4. In FIG. 15, the polyaxial bone screw with the locking device is shown without a tool. The first locking element 6 is screwed into the receiving part 4 and presses onto the free legs 55, 56 of the pressure element 5 to lock the head 3. The second locking element 7 presses onto the rod 100 to fix a position of the rod 100 in the channel. The rod 100 and the head 3 of the screw element 1 can be fixed independently from each other, which allows adjustments of the position of the rod 100 while maintaining an angular position of the screw element 1.

During surgery and before insertion of the locking device, the locking device is taken up by or engaged with the tool 101. By an operating error with respect to the tool, it may happen that the second locking element 7 is advanced, even if the locking device is not yet inserted into the receiving part 4. The first and second stops of the first locking element 6 prevent backing out or loss of the second locking element 7 from the first locking element 6. Also, when the locking device has already been inserted into the receiving part 4, subsequent adjustments that require a loosening of the fixation of the rod 100 can be safely performed because the second locking element 7 will abut against the first stop when it is screwed back, for example, by the first drive portion 102. Because the legs 55, 56 of the pressure element 5 extend above the rod 100, when the rod 100 is inserted, the first locking element 6 acts only onto the pressure element 5, and therefore only onto the head 3 and not the rod 100. The second locking element 7 acts only onto the rod 100.

A second embodiment of the locking device is shown in FIGS. 16 to 29. The second embodiment of the locking device can be used with the polyaxial bone screw as described before or with any other bone anchor. The locking device comprises a first locking element 600 and a second locking element 700. The first locking element 600 and the second locking element 700 are connected with the aid of a pin 800. The pin 800 is an abutment member that limits advancement or movement of the second locking member 700 in the first locking member 600.

Referring first to FIGS. 20 to 22, the first locking member 600 is substantially cylindrical with a first end 601, a second end 602, an external thread portion 603, and a coaxial bore 604. Adjacent the second end 602, an internal thread portion 605 is provided. An engagement structure 607 for engagement with a tool is provided in a region between the internal thread portion 605 and the first end 601. The engagement structure 607 can include extending recesses 607 similar to the first embodiment.

At a transition between the internal thread portion 605 and a portion of the bore 604 with the engagement structure 607, a circumferentially extending groove 606 is provided. The groove 606 may extend in a circumferential direction for slightly more than half a turn or slightly more than a semi-circle, as can be best seen in FIG. 22. An inner diameter d₅ of the groove is larger than an inner diameter d₆ of the internal thread portion 605 measured between the roots of the internal thread portion 605. A height of the groove 606 in an axial direction corresponds to at least one turn of the internal thread portion 605. By the groove 606, a first stop is provided by one end 606 c of the groove 606 in a circumferential direction and a second stop is provided by the other end 606 d in a circumferential direction of the groove 606.

The second locking element 700 has a first end 701, an opposite second end 702, and adjacent the first end 701 an externally threaded portion 703. A coaxial bore, i.e., recess 705 extends from the first end 701 in a direction of the second end 702. Adjacent the second end 702, a threadless portion 704 is provided. Adjacent the first end 701, a coaxial recess 706 with an engagement structure in the form of, for example, longitudinal recesses 707 arranged around a central axis C are provided for engagement with a tool. The coaxial engagement recesses 707 extend through the second end 702, thereby providing or forming holes 708. As can be seen in particular in FIGS. 23 to 25, a recess 709 is provided for accommodating the pin 800 seen in FIGS. 16 to 19. The pin 800 is cylindrical and fits into the recess 709 to such an extent that a portion 801 of the pin projects out of the recess 709. The pin 800 may be fixed to the recess. Furthermore, the pin 800 is sized so as to extend into the groove 606 as shown in FIGS. 26 to 29. The pin 800 forms an abutment member that is configured to abut against the first stop formed by one end 606 c of the groove 606 and against the second stop formed by the other end 606 d of the groove 606.

As shown in FIGS. 17 to 19, assembly of the locking device according to the second embodiment is carried out by introducing the second locking member 700 from the first end 601 of the first locking member 600. Once the second locking member 700 is nested into the first locking member 600 by engagement of the respective threads, the second locking member 700 is advanced towards the second end 602 of the first locking member 600 until the recess 709 is positioned at one of the recesses 607 of the engagement structure of the first locking member 600 that is at a position in the region of the groove 606. Then, as shown in FIG. 18, the pin 800 is inserted into the recess 709. Thereafter, the second locking element 700 can be rotated within the boundaries defined by the groove 606.

The functioning of the second embodiment of the locking device will be explained with reference to FIGS. 26 to 29. In FIG. 26, the second locking element 700 is in a first position. The outwardly projecting portion 801 of the pin 800 abuts against one end 606 c of the groove 606. The second end 702 of the second locking element 700 does not project out of the first locking element 600. In this preassembled state, the locking device can be delivered and used during surgery.

In FIG. 28, the second locking element 700 has been rotated and advanced downward until the outwardly projecting portion 801 of the pin 800 abuts against the other end 606 d of the groove 606. In this position, the second locking element 700 can no longer be advanced downward and therefore is prevented from backing out or escaping out of the first locking element 600. A length of the threadless portion 704 in an axial direction is such that in the second position, shown in FIG. 28, a portion of the second locking element 700 that projects outward from the first locking element 600 is suitable for pressing onto the rod 100 and fixing the rod 100.

As in the first embodiment, the external threaded portion 603 of the first locking element 600 can have any thread form, but preferably has a thread form that prevents splaying of the legs of a corresponding receiving part, in particular, a flat thread as shown, or a negative angle thread. The internal thread 605 of the first locking element 600 and the cooperating external thread 703 of the second locking element 700 may have a metric thread form or may also have a flat thread form, among others.

The materials of the parts and components of the locking device according to the second embodiment may be the same or similar to those of the first embodiment.

The second locking element 700 may also be introduced into the first locking element 600 from the second end 602. Since the second stop of the locking device in the second embodiment is located approximately in the middle of the first locking element 600 in an axial direction, the locking device according to the second embodiment can be designed to be more compact.

In a further modified embodiment, the first locking element 6′ includes an internal flat thread 63′ and the second locking element 7′ includes an external flat thread 73′ as can be seen from FIGS. 30 to 35 b. The thread form is the only difference between the first locking elements and the second locking elements according to the first embodiment and this further modified embodiment.

FIGS. 34 a and 34 b show the second locking element 7′ abutting against the abutment 66′ with its first end 71′.

FIGS. 35 a and 35 b show the second locking element 7′ abutting against the annular abutment 62 b′ with a portion of thread 73′ on a side opposite first end 71′.

With respect to the second embodiment, a modification to include the above mentioned flat thread configuration is also possible.

One advantage of the flat thread configuration, referring to the inner thread of the first locking element 6′ and the outer thread of the second locking element 7′ is that proper end stops are defined, because a flat thread is more robust and does not have sharp edges, which could violate an integral structure of the end stops. Because of the robust design of the flat threads, smaller dimensioning may be possible. This effect may also be intensified since no radial forces have to be taken up within the flat thread.

Further modifications of the embodiments described are also conceivable. A locking device according to embodiments of the invention can also be used with a monoaxial screw or with a bone plate. For example, the locking device can be used with a monoaxial screw and a rod that is made from an elastomeric material, which is used for dynamic stabilization.

The second locking element may have engagement structures at its second end that faces the rod, such as projections to enhance engagement with the rod.

Further, the engagement structures in the first and second locking element are not limited to the described grooves. They can have any other shape. Alternatively, an engagement structure at the surface of the first ends of the first and/or second locking element may be provided.

Other constructions of the stops for limiting the movement of the second locking element in the first locking element are also conceivable. For example, the first locking element may have an abutment member that abuts in a groove of the second locking member.

The independent head and rod fixation can also be realized by other means. For example, the pressure element can be shaped differently without legs that extend above the rod. In this case, the first locking element may have a structure at its second end that presses onto such a pressure element.

The polyaxial bone anchor can also be realized in many different known manners that are configured to engage with the locking device, for example, a polyaxial bone anchor with a favored angle bottom configuration, or with a bottom loader configuration.

While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is instead intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof. 

What is claimed is:
 1. A locking device for securing a rod-shaped element in a receiving part of a bone anchor, the locking device comprising: a first locking member comprising a first end, a second end, a central axis passing through the first end and the second end, an outer surface with an external thread, a coaxial bore passing through the first locking member, and an internal thread provided along at least a portion of the bore; and a monolithic second locking member comprising a first end, a second end, and an outer surface with an external thread configured to cooperate with the internal thread of the first locking member, wherein when the second locking member is positioned in the first locking member, the second locking member is movable along the central axis by rotating the second locking member relative to the first locking member, and wherein the first locking member or the second locking member comprises a first stop, such that movement of the second locking member is limited in a first direction along the central axis by the first stop, and wherein the first locking member or the second locking member comprises a second stop, such that movement of the second locking member is limited in a second direction along the central axis opposite to the first direction by the second stop.
 2. The locking device of claim 1, wherein when the first and second locking members are in an assembled state, the first and second locking members are inseparable.
 3. The locking device of claim 1, wherein when the second locking member is in a first position where the first and second locking members are in contact at the second stop, at least a portion of the second locking member protrudes outside of the bore of the first locking member.
 4. The locking device of claim 1, wherein the first locking member has an engagement structure at the first end of the first locking member and extending into the bore of the first locking member for engagement with a tool.
 5. The locking device of claim 1, wherein the first stop comprises an abutment provided on the first locking member.
 6. The locking device of claim 1, wherein the second stop comprises an abutment at the second end of the first locking member.
 7. The locking device of claim 1, wherein the coaxial bore passes entirely through the first locking member.
 8. The locking device of claim 1, wherein the greatest width of the second locking member measured radially is less than or equal to the greatest width of the coaxial bore of the first locking member.
 9. The locking device of claim 1, wherein the second locking member is configured to be located entirely within the coaxial bore of the first locking member.
 10. A bone anchor comprising: an anchoring element comprising a head and a shank to be anchored to a bone or a vertebra; a receiving part configured to be connected to the shank, the receiving part comprising: a first end opposite to the shank and a second end facing the shank when the anchoring element and the receiving part are assembled; a longitudinal axis passing through the first and second ends; a bore coaxial with the longitudinal axis and extending from the first end of the receiving part through at least a portion of the receiving part; and a substantially U-shaped recess for receiving a rod-shaped element, the recess defining two free legs extending in a direction of the first end of the receiving part, wherein the legs are provided with an internal thread; a pressure element configured to be positioned in the bore of the receiving part and to exert pressure onto the head of the anchoring element; and a locking device comprising: a first locking member comprising a first end, a second end, an outer surface with an external thread configured to engage the internal thread of the legs, a coaxial bore passing through the first locking member, and an internal thread provided along at least a portion of the bore of the first locking member; and a monolithic second locking member comprising a first end, a second end, and an outer surface with an external thread configured to engage the internal thread of the first locking member; wherein when the second locking member is positioned in the first locking member, the second locking member is movable by rotating the second locking member relative to the first locking member, and wherein movement of the second locking member is limited in a first direction by a first stop and in a second direction opposite to the first direction by a second stop; wherein when the pressure element and the locking device are in the receiving part, the first locking member or the second locking member is configured to contact the pressure element.
 11. A locking device for securing a rod-shaped element in a receiving part of a bone anchor, the locking device comprising: a first locking member comprising a first end, a second end, a central axis passing through the first end and the second end, an outer surface with an external thread, a coaxial bore passing through the first locking member, and an internal thread provided along at least a portion of the bore; and a monolithic second locking member comprising a first end, a second end, and an outer surface with an external thread configured to cooperate with the internal thread of the first locking member, wherein when the second locking member is positioned in the first locking member, the second locking member is movable along the central axis by rotating the second locking member relative to the first locking member, wherein movement of the second locking member is limited in a first direction along the central axis by a first stop and in a second direction along the central axis opposite to the first direction by a second stop; and wherein the external thread of the second locking member is a flat thread. 